lnwallet+docs: minrelayfee always above fee floor

The minimum relay fee is always ensured to be above our fee floor except
in the very first min relay fee query to bitcoind. This commit ensures
that the fee floor is respected in this first query.

lnwallet/chainfee/minfeemanager.go

@@ -32,6 +32,12 @@ func newMinFeeManager(minUpdateInterval time.Duration,
 		return nil, err
 	}
 
+	// Ensure that the minimum fee we use is always clamped by our fee
+	// floor.
+	if minFee < FeePerKwFloor {
+		minFee = FeePerKwFloor
+	}
+
 	return &minFeeManager{
 		minFeePerKW:       minFee,
 		lastUpdatedTime:   time.Now(),